Exercise Solutions Introduction

Slide 13

1. list can store different data types
2. data frames can store different values with different types
3. class()
4. is.character()

Slide 14

• [] = always returns object of same class
• [[]] = can extract one element from list or data frame
• list_a <- list(a = 2, b = "hallo")
  • list_a["a"]
  • list_a[["a"]]
  • list_a$a

• [ selects sub-lists. It always returns a list; if you use it with a single positive integer, it returns a list of length one.
• [[ selects an element within a list.
• $ is a convenient shorthand: x$y is equivalent to x[["y"]].

• Use drop = FALSE if you are subsetting a matrix, array, or data frame and you want to preserve the original dimensions. You should almost always use it when subsetting inside a function.

Slide 15

1. • if() works with scalars; ifelse() works with vectors.
2. • when x is TRUE, y will be 3;
   • when FALSE, y will be NULL;
   • when NA the if statement will throw an error.

• You can rewrite any for loop to use while instead, and you can rewrite any while loop to use repeat, but the converses are not true. That means while is more flexible than for, and repeat is more flexible than while.

if("2") 3
if(2) 3

for(i in 1:3){
  print(i)
}

i <- 1
while(i < 4){
  print(i)
  i <- i + 1
}

i <- 1
repeat{
  print(i)
  i <- i + 1 
  if(i > 3) break
  
}